The present invention relates to a brushless DC motor and, more particularly, relates to a control system of the brushless DC motor which is suitable for rotating precision apparatus, for example, a magnetic disk in a magnetic disk apparatus, without a magnetic sensor.
In conventional brushless DC motors, a magnetic sensor such as a Hall element is employed, instead of a brush, to detect a rotor position relative to a stator. A coil or phase to which a current is made to flow is switched over in accordance with the detected rotor position.
However, control methods of the brushless DC motor wherein the rotor position is detected without using the magnetic sensor have been proposed.
As is described in the Japanese Patent Laid-Open No. 59-162793, for instance, a method has been proposed wherein a counterelectromotive voltage signal generated in a coil with the rotation of a field rotor of the brushless DC motor is used to detect the rotor position so as to determine the timing of making a driving current flow to each coil.
Moreover, as is described in the EPC Application Laid-Open No. 251785, a starting method has been proposed wherein a high-frequency current forming a short current pulse is made to flow through each coil while the motor is rotated. A peak amplitude value of this current is measured to detect the rotor position so as to determine the timing of making the driving current flow to each coil. Also a second method is proposed for switching when the current rises above a value that changes with speed. For constant speed rotation, current is varied by detecting speed according to timing marks on a rotated disk.